When Wolfram Research's FinancialData was in its initial Experimental stage and available in Mathematica, I recall that CUSIP and possibly ISIN were available data elements.
Those data elements are no longer available in the public version of FinancialData.
I don't have a need for the FinancialData Platform that Wolfram Research provides through subscription or contract, but maybe those identifiers are available on that platform. So I would suggest contacting them.
I believe that Xignite was in the initial list of data provider for some FinancialData data source. Xignite is not free. Who are the remaining data providers? If it is Yahoo Finance, then https://help.yahoo.com/kb/learn-find-symbol-sln2340.html.
Yahoo Finance states in the linked document that it is not designed for ISIN search. Don't know if that is true in other jurisdictions.
I believe you are looking for a ISIN to Ticker Symbol master file. A topic that many financial researchers often face and sometimes build internally even when they can get some of the data from paid data providers. see https://medium.com/prooftrading/proof-engineering-security-master-4e2ac164511f
If the financial instruments you require data are mostly international, but some how need to file with the SEC, then there could be some data available in some the SEC form filings.
There seems to be a perception of transparency in financial data in some countries, but simply having ISIN (Instrument Identifier) map to company/issuer/obligator (Issuer Identifier) is not proving to be transparent for your needs. You need the Symbol.
Also there are many impediments, at least in the US, that keep CUSIP which is a basis for US based (company has a headquarter in US) instruments from use and storage without a license.
And some ISIN are derived from CountryCode+CUSIP+CheckDigit.
See https://rosettacode.org/wiki/Validate_International_Securities_Identification_Number
Mathematica solution for validating ISIN.
Hopefully you can find a solution that does not break any licensing issues.
Hans
